istream/head: fix infinite recursion in _Skip()
istream/head: fix infinite recursion in _Skip()

Calling Istream::Skip() will look up HeadIstream::_Skip() in the
vtable, not ForwardIstream::_Skip().

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy